1. Field of Invention
This invention relates, in general, to a method of manufacturing a triple level read only memory (ROM) device, and to a ROM device manufactured using the method.
2. Description of Related Art
ROM devices are widely used in digital equipment, such as micro-computers and microprocessor systems, for storing vital system information. The known methods of manufacturing ROM devices are complicated, time consuming and expensive to implement. Hence, customers typically submit a program code to the ROM factory and let the factory code the program into their ROM products.
Most ROM components, aside from differences in the code stored during the programming stage, have basically the same physical structure. Therefore, the ROM device can be manufactured up to a stage just prior to the performing of the actual programming, and stored as a partially finished product in a warehouse. When the customer places an order for a ROM requiring a certain program code, a photomask can be immediately manufactured, so that programming can be performed on short notice. Photomask programming of pre-manufactured ROM devices is widely used in the industry.
Typically, a channel transistor is used as the basic memory cell unit of the ROM device. During the programming stage, ions are selectively implanted into specified channels, thus adjusting the threshold voltage and achieving an ON/OFF control state of the memory unit, as stipulated by the coding process.
FIG. 1 is a top view of ROM memory cell units 100, 101, 102 and 103 in an integrated circuit. In FIG. 1, polysilicon bit lines (BL) extend across word lines (WL). A channel 10 is formed, for example, in the memory cell unit 100, in an area beneath the bit line and where the bit line crosses the word line. A bit "0" or "1" is stored in the binary level memory cell unit 100, depending on whether or not ions have been implanted into the channel 10.
Products having a binary level configuration, as described above, have a restricted memory capacity. As such, coding techniques using three or more levels have been suggested. At present, a multiple coding implantation method is used in the fabrication of multiple level ROM units to generate different threshold voltages on different memory cell units.
FIG. 2 is a graph showing the relationship between different threshold voltages V.sub.T, V.sub.T1 and V.sub.T2 versus the drain current, of a known ROM device having multiple level logic states. The threshold voltages are generated by the selective implantation of ions into the channels of the memory cell units. However, this known method requires that the number of coding implants be increased to produce the multiple level ROM device. As such, production time is increased, thus decreasing the competitiveness of a product made using this method.